1. Field of the Invention
The present invention relates to a water-based pigment-type ink that can be preferably used for an ink-jet recorder, and to an ink-jet printer for multi-color printing, using the water-based pigment-type inks.
2. Description of the Related Art
Ink-jet printers have been extensively used for effecting printing by ejecting inks through nozzles of very small diameters. According to this recording system, the liquid ink is ejected onto a recording medium as flying particles from nozzles (orifices) of a very small diameter, so that flying particles of the ink adhere onto the recording medium to thereby form characters, images and the like.
An ink-jet printer of this type generally employs inks of the type obtained by dissolving various dyestuffs in water or in an organic solvent. However, the dyestuff inks are inferior in regard to fastness to light and water resistance, and are not suited where the recorded image requires fastness to light or water resistance.
In recent years, therefore, it has been proposed to use pigment inks instead of dyestuff inks. When pigment-type inks are to be used for the ink-jet printer, it has been proposed to add particular additives to pigment-type inks or to prepare pigment-type inks in particular compositions to improve the dispersion stability. That is, in the pigment-type inks, the pigment components coagulate or settle in the ink solutions, since the pigments exhibit poor affinity to the water. Upon adding the additives or forming the inks in particular compositions, however, it becomes possible to suppress the coagulation or settlement.
Japanese Unexamined Patent Publication (Kokai) No. 7-292302 discloses a recording solution used for a device for effecting a non-contact non-impact recording by ejecting an ink containing pigment particles. This publication teaches that a difference C (=A-B) of weight between a nonvolatile component (A) and a color component (B) should be 0.01 C&lt;10 in the recording solution at 25.degree. C. This publication further discloses the use of a plurality of color inks inclusive of a black ink, and specifies the ratio (=B/A) of the nonvolatile component (A) to the color component (B) to be not larger than 0.5 in the recording solutions at 25.degree. C.
Furthermore, WO95/21897 discloses a water ink composition containing a water-insoluble coloring material, a water-soluble organic solvent and water, without substantially containing a dispersant, said water-insoluble coloring agent having a solubility in water of not larger than 1% by weight at 20.degree. C. but being soluble in said water-soluble organic solvent, and said water-insoluble coloring agent being dispersed in the ink composition.
Thus, pigment-type inks have heretofore been proposed having ink compositions in which the amount of the additive which is a dispersant is set to a particular ratio, or the solubility is set to lie within a particular range without substantially containing dispersant.
As described above, the dyestuff or the pigment has heretofore been used by being dissolved in, and decomposed by, a high-boiling solvent and water. However, the dyestuff involves a problem in regard to fastness to light and water resistance, and the pigment involves a problem in regard to coagulation and settlement.
The pigment-type ink is superior to the dyestuff-type ink in regard to fastness to light and water resistance, but permits a pigment component having poor affinity to water to be coagulated or settle in the ink solution. Therefore, when the conventional pigment-type ink is used directly for the ink-jet printer, the ink is ejected from the nozzles (orifices) of the ink-jet head lacking stability to a conspicuous degree, and the printing becomes defective. This tendency is observed by either the type using heat energy or the type using pulse pressure; i.e., an ink residue formed near the orifices prevents stable ejection of the ink.
The ink-jet printer for multi-color printing uses a plurality of color inks inclusive of a black ink. That is, there are used black, yellow, magenta and cyan inks. In recent years, carbon black has been used as a pigment for a black ink owing to its excellent fastness to light and water resistance. However, when the carbon black is used in the ink-jet printer, black appears to be somewhat reddish brown when black exists among other colors, causing the quality of multi-color print to be deteriorated.